Recently, in a hydraulic brake system of a vehicle, for example, as described in the following Patent Document 1, a master cylinder device that is normally actuated so as to pressurize a brake fluid depending only on a pressure of the highly pressured brake fluid which is introduced from a high pressure source, is employed. The master cylinder device commonly has a stroke simulator. According to an operation force applied to an operation member by a driver, the stroke simulator allows a movement of the operation member while generating an operation reaction force against the operation force. Therefore, even when the brake fluid is not pressurized depending on the operation force, the driver can realistically feel the operation reaction force while moving the operation member by a driver's brake operation. Thus, the stroke simulator can make operational feeling in a brake operation be favorable even when the master cylinder device pressurizes the brake fluid depending only on the pressure of the brake fluid highly pressurized by the high pressure source. However, under a condition in which the high pressure source cannot work normally due to an electric failure, etc., the master cylinder device cannot be actuated so as to pressurize the brake fluid depending only on the pressure of the brake fluid highly pressured by the external high pressure source. For this reason, the master cylinder device also has a function for pressurizing the brake fluid depending only on the operation force of the driver.
Patent Literature 1: JP-A-2-279450